The present invention relates generally to integrated circuit amplifier circuits, and more particularly to operational amplifiers capable of delivering high output currents, and more particularly to avoiding high power dissipation in the output stage of a high-output-current operational amplifier. The invention also relates to avoiding use of additional voltage source circuits and associated large capacitances for providing constant voltages to the gates of high-voltage output cascode transistors of the output stage of the operational amplifier.
FIG. 1 illustrates a prior art integrated circuit operational amplifier 1A in which some of the transistors, including the output stage cascode transistors 29 and 30, are high-voltage transistors used to provide high voltage operation, other transistors of the operational amplifier being low-voltage transistors which are used to provide high speed and high bandwidth operation. The structure of the circuit of FIG. 1 is disclosed in commonly owned U.S. Pat. No. 6,657,495 issued Dec. 2, 2003 to Ivanov et al., and its operation is partly described in that patent and also is further described in another commonly owned U.S. Pat. No. 6,150,883 issued Nov. 21, 2000 to Ivanov that is incorporated by reference into U.S. Pat. No. 6,657,495.
Input stage 21, folded cascode stage 45, and class AB bias circuit 46 of FIG. 1 are the same as in subsequently described FIG. 3. The drain of cascode transistor 13 in FIG. 1 is connected by conductor 25 to the gate of a P-channel output transistor 27 and output stage 49A. The drain of cascode transistor 15 in folded cascode stage 45 is connected by conductor 26 to the gate of a N-channel output transistor 28.
Output stage 49C includes above-mentioned P-channel output transistor 27 and N-channel output transistor 28. The source of output transistor 27 is connected to VDD, and the source of output transistor 28 is connected to ground. The drain of output transistor 50 is connected by conductor 50 to the source of a P-channel output cascode transistor 29, the drain of which is connected by Vout conductor 31 to produce the output voltage Vout. Similarly, the drain of output transistor 20 is connected by conductor 60 to the source of N-channel output cascode transistor 30, the drain of which is connected to conductor 31. A constant voltage source 32 is connected between VDD and the gate of output cascode transistor 29 to provide a constant gate voltage thereon, and a voltage source 33 is connected between ground and the gate of output cascode transistor 30 to provide a relatively constant gate-to-source voltage on it. Relatively large capacitors C1 and C2 are connected across voltage sources 32 and 33 as shown to effectively maintain the needed constant gate-to-source voltages of output cascode transistors 29 and 30.
During large-signal excursions of Vout, the parasitic capacitance from the source to the gate of output cascode transistors 29 and 30 can result in large peak currents. If the voltage sources 32 and 33 are unable to maintain the gate voltages of output cascode transistors 29 and 30 at relatively constant values, the result can be undesirable signal distortion in Vout.
In operational amplifier 1A of FIG. 1, voltage sources 32 and 33 need to have low impedance at high frequencies, and in low-current circuits this is achieved by providing the capacitors C1 and C2 to produce a low value of high-frequency impedance at the gates of output cascode transistors 29 and 30 so that output cascode transistors 29 and 30 do not introduce appreciable delay in the signal path of such low-current operational amplifier circuits.
However, high-current implementations of the operational amplifier circuit 1A of voltage sources 32 and 33 also need to have low values of high-frequency impedance in order to provide high-bandwidth. The high frequency impedance referred to should be substantially lower, roughly 5 to 10 times smaller, than the gate impedances of output cascode transistors 29 and 30 to avoid signal distortion in Vout. It should be appreciated that high impedance at the drains of output transistors 27 and 28 can cause distortion in Vout. High impedance of the voltage sources 32 and 33 in prior art FIG. 1 during signal transients causes signal changes at the drains of output transistors 27 and 28. This causes signal distortion because of parasitic feedback through gate-drain capacitances of output transistors 27 and 28.
Unfortunately, in order to achieve the needed low values of high-frequency impedance in a high-current operational amplifier circuit having the construction of prior art FIG. 1, the capacitance of capacitors C1 and C2 needs to be so large that it is unfeasible to include them on the same integrated circuit chip. This is undesirable because the large amount of capacitance required for high-current applications makes it impractical to provide capacitors C1 and C2 on the same integrated circuit chip as the rest of the operational amplifier circuitry. This is highly undesirable to most customers, and therefore it has been impractical to use the structure of FIG. 1 in a state-of-the-art high-current integrated circuit operational amplifier.
FIG. 2 illustrates an amplifying circuit 1B including an output stage 49B which is capable only of sinking current from a load (not shown). Output transistor 28 and output cascode transistor 30 are the same as in prior art FIG. 1, but voltage source 33 and capacitor C2 of FIG. 1 have a been replaced by a gate control amplifier circuit 67. Gate control amplifier circuit 67 includes a N-channel transistor 61 having its source connected to ground and its drain connected by conductor 63 to the gate of output cascode transistor 30 and to one terminal of a current source 68, the other terminal of which is connected to VDD, wherein conductor 63 functions as the output of gate control amplifier 67.
The amplitude of the current source 68 is necessarily larger than the maximum instantaneous gate current of output cascode transistor 30. Since the magnitude of the current source 68 would be that large for a high-current output stage 49B, the result of using the current source 68 would be an unacceptably large increase in the overall power consumption of the amplifying circuit 1B if it is designed to sink large output currents.
No solution to the foregoing problems of high current implementations of the circuits of prior art prior art FIGS. 1 and 2 have been provided, so it has been impractical to use the output stage circuit structures of FIGS. 1 and 2 in high-current integrated circuit operational amplifiers or in output stages of other kinds of amplifier circuitry.
Thus, there is an unmet need for an amplifier including an output circuit which is capable of providing high output current without the need for using the large capacitors of prior art FIG. 1.
There also is an unmet need for an amplifier including an output circuit which is capable of providing high output current without the need for providing the voltage sources 32 and 33 required in the circuit of prior art FIG. 1.
There also is an unmet need for an amplifier including an output circuit which is capable of providing high output current without high power dissipation required using the circuit of prior art FIG. 2.
There also is an unmet need for an amplifier including an output circuit which is capable of providing high output current with reduced signal distortion.